1. Field of the Invention:
The present invention relates to earth boring bits and specifically to a welding method and apparatus for welding a multiplicity of individual sections together to form a rock bit.
2. Description of the Prior Art:
A rotary rock bit in general consists of a bit body made by welding separate sections together along seams between the sections. The body of a typical, contemporary bit is constructed of three sections, assembled to engage across 120 degree, machines faces and welded to form an integral body unit. A cone is mounted, prior to welding, on a cantilevered bearing shaft that extends from each section. The assembled bit has a connecting end for connection to a string of drill pipe for rotation about a vertical axis.
In recent years, attention has been focused on improvements to the welding and assembly procedures to improve the geometry, the dimensional accuracy and the design integrity of the rock bit. In the prior art method of manufacturing rotary rock bits, a weld groove was formed between adjacent sections of the bit and the weld groove was filled with a weld deposit by a welding process. Surfaces on adjoining sections were in adjacent relationship but the surfaces were not joined together and the joining of the sections was through the weld deposit. In order to correctly join the individual sections of the bit body together, they must be accurately positioned during the welding process.
A method to improve the accuracy of the assembly and welding of the sections of a rock bit is shown in U.S. Pat. No. 4,209,124, Rock Bit Assembly Method, June 24, 1980. Here, a shank positioner and a fixed ring are positioned with a fixture to hold the sections and assembled cones to minimize slippage of the sections during assembly and welding.
Another method to improve the integrity in rock bit design parameters is disclosed in U.S. Pat. No. 4,414,734, Triad For Rock Bit Assembly, Nov. 15, 1983. A specially shaped dowel called a triad is used to maintain the 120 degree faces of the sections in the intended relationship. This method further assures accuracy of alignment of the sections during assembly and welding.
In spite of these improvements, certain deficiencies were inherent in the prior art welding techniques, including the automated or robotic welding techniques. In the typical prior art technique, the bit sections were chucked in a fixture and weld grooves were formed by machining. In other prior art techniques, the weld grooves were cut along the radial and axial seams by a robot equipped with an air carbon arc cutting torch. This procedure was repeated for each bit in the group of bits being manufactured. The tooling would then be changed on the robot to permit welding with a gas shielded flux-cored-arc welding torch. Each bit would then be rechucked, in turn, and the radial and axial seams would be welded. The multiple chuckings of the bits during robotic assembly and welding have caused repositioning errors which can result in weld flaws such as lack-of-fusion defects, poor tie-ins, and spill overs.